Martin H. Studier

Origin of organic matter in the early solar system—VII. The organic polymer in carbonaceous chondrites

Abstract The insoluble polymer from the Murchison C2 chondrite was studied by a variety of degradation techniques: pyrolysis, depolymerization by Na 4 P 2 O 7 or CF 3 COOH, and oxidation by HNO 3 , Na 2 Cr 2 O 7 , or O 2 /UV light. Products were identified by IR spectroscopy, gas chromatography, and mass spectrometry (time-of-flight and high-resolution). In some cases, parallel measurements were made on a synthetic polymer produced by the Fischer-Tropsch reaction, a meteoritic polymer from the Allende C3V chondrite, and samples of coal or related materials. Our studies confirm the prevailing view that the meteoritic polymer has a bridged aromatic structure with functional groups such as COOH, OH, and CO, but provides much new detail. Oxidation with HNO 3 shows that the meteoritic and synthetic polymers have a similar degree of condensation, greater than that of high-volatile bituminous coal. Gentler oxidation with Cr 2 O 2− 7 or O 2 /UV led to the identification of 15 aromatic ring systems as the corresponding carboxylic acids: benzene, biphenyl, naphthalene and phenanthrene and their methyl derivatives, fluoranthene (or pyrene), chrysene, fluorenone, benzophenone, anthraquinone; and the heterocyclics dibenzofuran, benzothiophene, dibenzothiophene, pyridine, quinoline (or isoquinoline), and carbazole. Of 11 aliphatic acids identified, three dicarboxylic acids presumably came from hydroaromatic portions of the polymer, whereas eight monocarboxylic acids probably are derived from bridging groups or ring substituents. Depolymerization with CF 3 COOH yielded some of the same ring systems, as well as alkanes (C 1 –C 8 ) and alkenes (C 2 –C 8 ), alkyl (C 1 –C 5 ) benzenes and naphthalenes, and methyl- or dimethyl-indene, -indane, -phenol, -pyrrole, and -pyridine. All these compounds were detected below 200°C, and hence probably were indigenous constituents rather than pyrolysis products. Though the match between the synthetic and meteoritic polymer is only fair, several properties of the latter suggest that it, too, was produced by surface catalysis: the predominance of n -alkyl fragments, its occurrence as a surface coating on specific kinds of mineral grains, and the C 13 C 12 fractionation between polymer and coexisting carbonates.

Organic Compounds in Meteorites: They may have formed in the solar nebula, by catalytic reactions of carbon monoxide, hydrogen, and ammonia.

Organic compounds in meteorites seem to have formed by catalytic reactions of CO, H2, and NH3 in the solar nebula, at 360� to 400�K and (4 to 10) x 10-6 atm. The onset of these reactions was triggered by the formation of suitable catalysts (magnetite, hydrated silicates) at these temperatures. These reactions may be a source of prebiotic carbon compounds on the inner planets, and interstellar molecules.

Origin of organic matter in early solar system. V - Further studies of meteoritic hydrocarbons and a discussion of their origin.

Abstract The Murchison meteorite contains aliphatic and aromatic hydrocarbons similar to those made in static Fischer-Tropsch-type syntheses. Principal compound classes above C8 are n-alkanes, mono- and dimethylalkanes, alkenes, alkylbenzenes and -naphthalenes. Below C8, n-alkanes are virtually absent; instead, benzene, toluene, branched alkanes dominate. The CH4/C2H6 ratio is greater than 30, possibly greater than 700. Isoprenoids from C17 to C20 occur in a surface rinse but not in subsequent extracts and appear to be terrestrial contaminants. Thiophenes, porphyrin-like pigments and chlorobenzenes were also found; the latter appear to be contaminants. In the Allende meteorite, only methane, benzene, toluene and an aromatic polymer seem to be indigenous. A comprehensive review of current evidence shows that Fischer-Tropsch-type reactions can account for most principal features of meteorite organic matter: hydrocarbons, nitrogen bases, amino acids, carbon-isotope fractionations and possibly pigments. Some features (aromatic hydrocarbons) require a reheating stage; others such as S- and Cl-compounds and the aromatic ‘polymer’ remain unexplained and need to be investigated. No other process is known at present that accounts for an equally wide range of evidence.

Trapped organic compounds and aromatic units in coals

Abstract Some insight into the chemical nature of coals and the coalification process was obtained by detailed analyses of the organic constituents of three coals — a lignite, a bituminous, and an anthracite coal. Organic compounds trapped in the coal matrix, residuals and products of the original coalification process, were isolated by vacuum distillation and solvent extraction. The macromolecular material which constitutes the bulk of coals was degraded by a series of selective oxidations to smaller units which could be identified and measured. The essential aromatic character of coals was demonstrated, with condensation of aromatic rings increasing with increasing rank.

Thiobases in Escherchia coli Transfer RNA: 2-Thiocytosine and 5-Methylaminomethyl-2-thiouracil

Two new sulfur-containing pyrimidine nucleotides have been isolated from hydrolyzates of Escherichia coli transfer RNA. The structures, 2-thiocytosine and 5-methylaminomethyl-2-thiouracil, have been assigned to the bases as a result of study of ultraviolet and mass spectra. An acid-degradation product, S-methylamino-methyluracil, has been synthesized and is identical to that derived from the natural product.

Origin of organic matter in early solar system. I - Hydrocarbons.

Abstract Organic compounds in the Orgueil and Murray carbonaceous chondrites were examined by combination gas chromatography/mass spectrometry. Above C 10 , normal paraffins are the principal species, with lesser amounts of 2-methyl-, 3-methyl-, and other slightly branched paraffins and olefins. Below C 10 , aliphatic hydrocarbons are markedly deficient, while benzene and alkylbenzenes dominate. COS, CS 2 , m - and p -dichlorobenzene were also seen in both meteorites. Three isoprenoid hydrocarbons (C 11 , C 13 , C 14 ) were tentatively identified in Murray. A similar hydrocarbon distribution was synthesised from CO and D 2 in the presence of iron meteorite powder. This reaction (essentially a Fischer-Tropsch synthesis) yields a metastable distribution of normal and slightly branched paraffins and olefins, including isoprenoid hydrocarbons from C 9 to C 19 . Sustained reheating causes partial transformation of aliphatics to aromatics, the degree of conversion depending on time and temperature. At 900°C, polynuclear aromatic hydrocarbons such as pyrene, benzopyrene and coronene were obtained in yields of 1–8 per cent. Catalytic reactions of this type may have taken place on a large scale in the solar nebula, converting CO to less volatile carbon compounds, and thus enabling carbon to condense in the inner solar system. Further opportunities for hydrocarbon synthesis would be provided during impact of carbon-bearing planetesimals or comets on the Earth, Moon and Mars.

Purines and triazines in the Murchison meteorite

Abstract Two samples of the Murchison C2 chondrite were examined for organic nitrogen compounds, using mass spectrometry as well as paper and thin-layer chromatography. Under mild extraction conditions (water or formic acid) only aliphatic amines and C 2 -C 6 alkylpyridines were seen; the latter may be contaminants. Under drastic extraction conditions (hot, 3–6 M HCl or CF 3 COOH), a variety of basic nitrogen compounds appeared, in the following amounts (ppm): adenine (15), guanine (5), melamine (20), cyanuric acid (20–30), guanylurea (30–45), urea (25), etc. Apparently these compounds are present mainly in macromolecular material, and are released only upon acid hydrolysis. These findings support our earlier identifications of these compounds in the Orgueil meteorite. They also suggest that the recent failure by Folsome et al. ( Nature 232 , 108–109, 1971; Geochim. Cosmochim . Acta 37 , 455–465, 1973) to find purines or triazines in carbonaceous chondrites was due to inadequate extraction conditions: water and formic acid, rather than HCl. Conversely, we were unable to detect the principal compound class reported by Folsome et al. : 4-hydroxypyrimidines.

Origin of organic matter in early solar system—II. Nitrogen compounds

Abstract Previous identifications of nitrogen compounds in the Orgueil meteorite were confirmed. Guanylurea was identified as an additional, major constituent (270 ppm). All of the nitrogen compounds seen in carbonaceous chondrites (adenine, guanine, melamine, ammeline and guanylurea) form spontaneously, in yields of 0·1–0·5%, when CO, H 2 and NH 3 are allowed to react in the presence of iron meteorite powder. Cytosine, cyanuric acid, biuret and urea also form, along with an assortment of hydrocarbons similar to those in meteorites. These results support the suggestion of Studier, Hayatsu and Anders that the organic compounds in meteorites formed in the solar nebula, by spontaneous reactions of CO, H 2 and NH 3 . A large part of the prebiotic organic matter on the Earth may have originated in a similar manner.

Carbynes in meteorites - Detection, low-temperature origin, and implications for interstellar molecules

Carbon from the Allende meteorite is not graphite but carbyne (triply bonded elemental carbon), inasmuch as on heating to 250� to 330�C it releases mainly triply bonded fragments: –(C≡C)n,– with n = 1 to 5, and –(C≡C)n–CN, with n = 1 to 3. Although carbynes have been known to form only by condensation of carbon vapor above 2600 K or by explosive shock of > 600 kilobars, it is found that they also form metastably by the reaction 2CO → CO2 + C (solid) at 300� to 400�C in the presence of a chromite catalyst. Such low-temperature formation by surface catalysis may be the dominant source of carbynes on the earth and in meteorites, and a major source of interstellar carbynes and cyanopolyacetylenes.

Phenolic ethers in the organic polymer of the Murchison meteorite

Seven phenolic acids and many nonphenolic organic acids, including large amounts of meta-hydroxy (3-hydroxy) benzoic acid and 3-hydroxy-1,5-benzene-dicarboxylic acid, were obtained from the organic polymer of the Murchison C2 chondrite upon oxidation with alkaline cupric oxide. The phenolic acids apparently were derived from phenolic ethers in the polymer, which in turn probably were formed from carbon monoxide and hydrogen by catalytic Fischer-Tropsch type reactions in the solar nebula. In contrast, terrestrial polymers such as lignin, humic acid, and coal yield mainly para-hydroxy (4-hydroxy) benzene derivatives by the same oxidation procedure.